A handy phrase meaning that the insult of being called a stupid arsehole, or (an alternative interpretation) an asinine fool, can be doubled and can be reflected back to whence it came in a way that turns a totally uncalled for insult flung at its intended target back upon its perpetrator, possibly also in a way not fully understood by its vulgarian receiver, he not being up on such things as witty retorts, or such logical insinuations as this phrase supposedly represents. However, if no other rebuffs spring to mind, in a pinch it does the job nicely.

Watch what you say to me, you double-dumbass, as I wasn’t even talking to you, I was talking to myself!